750=9t+4.9t^2

Solution for 750=9t+4.9t^2 equation:

750=9t+4.9t^2

We move all terms to the left:

750-(9t+4.9t^2)=0

We get rid of parentheses

-4.9t^2-9t+750=0

a = -4.9; b = -9; c = +750;
Δ = b2-4ac
Δ = -92-4·(-4.9)·750
Δ = 14781
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{14781}=\sqrt{1*14781}=\sqrt{1}*\sqrt{14781}=1\sqrt{14781}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-1\sqrt{14781}}{2*-4.9}=\frac{9-1\sqrt{14781}}{-9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+1\sqrt{14781}}{2*-4.9}=\frac{9+1\sqrt{14781}}{-9.8}
$